1. Field of the Invention
The present invention relates to processes and equipment for manufacturing semiconductors, such as solar cells; and more particularly to apparatus and methods for aligning a component of the manufacturing equipment with a wafer of material on which the semiconductors are being fabricated.
2. Description of the Related Art
Semiconductor devices, such as photovoltaic devices that are commonly called solar cells, are fabricated on a wafer of silicon or other material. Various sections of the wafer are electrically interconnected by a pattern of silver or other conductive material deposited on a surface of the wafer. Because of the relatively large size of the photovoltaic elements, a screen printing process, similar to silk screen printing, typically is employed to deposit the silver in the proper pattern on the wafer surface. The printing screen used in that deposition process must be precisely aligned with the wafer in order that the silver conductive pattern is properly registered with the semiconductor devices.
Unlike the processes for manufacturing other types of semiconductors, such as integrated circuits, the production of photovoltaic devices often is not performed in highly clean conditions. High volume, low cost production techniques and equipment frequently are used. As a result, the manufacturing process may leave a residue on the photovoltaic devices, which would be unacceptable for other types of semiconductors.
Machine vision systems are commonly used in semiconductor device processing. In such systems, a video camera produces an image of the semiconductor wafer and that image is then analyzed for various purposes, such as defect detection. In other situations, the wafer image is utilized to determine whether the wafer is properly positioned on a work surface of a processing apparatus. For example, the registration of the wafer and the printing screen is verified with a camera that produces an image of the wafer with the printing screen there behind.
Difficulty has been encountered when attempting to utilize conventional machine vision systems to align the printing screen used to deposit the conductive pattern on a wafer of photovoltaic devices. Such systems illuminate the wafer. In a direct illumination technique, light is projected from the same side of the wafer as the camera. This technique did not always provide sufficient contrast between the wafer and the printing screen to enable the vision system to reliably and accurately detect the edges of the wafer. Due to the residue on the wafer, both the printing screen and the wafer reflected visible light similarly, which adversely affected the ability to distinguish between those objects. One prior solution used ultra-violet light, however certain types of residues still precluded sufficient contrast between the wafer and the printing screen.
An alternative illumination technique involved backlighting the printing screen, however placing a light source on the side of the printing screen remote from the wafer interfered with the printing process.
As a consequence, it is desirable to develop an alternative technique for illuminating semiconductor wafers when using a machine vision system.